Oscillator stability monitoring and compensation system

Abstract

The invention includes analyzing the steering voltage applied to a crystal oscillator over time, and compensating for spurious frequency jumps in determining the drift rate of a crystal oscillator. The steering voltage may be used to estimate oscillator stability by comparing a projected steering voltage against an actual voltage after a simulated holdover period, or analyzing a steering voltage recorded over a period of time and evaluating rates of change. Spurious frequency jumps may be removed from data collected while not in an actual holdover, making the data more accurately represent the frequency drift rate of the oscillator. Also, the rate of occurrence of spurious frequency jumps while not in holdover may be monitored to provide information regarding the physical condition of the crystal.

We claim:  
     
       1. A method of testing stability of an oscillator, comprising: 
       characterizing oscillator stability relative to a system reference clock;  
       operating the oscillator independent of the system reference clock for a period of time to simulate holdover;  
       deriving a holdover error that is the difference between the system reference clock and the oscillator over the period of time;  
       detecting spurious frequency jumps in the oscillator while characterizing the oscillator stability; and  
       compensating for the spurious frequency jumps in characterizing the oscillator stability.  
     
     
       2. The method of claim  1 , wherein characterizing oscillator stability relative to a system reference clock comprises characterizing an oscillator steering voltage over time. 
     
     
       3. The method of claim  2 , further comprising: 
       detecting spurious frequency jumps while characterizing an oscillator steering voltage; and  
       compensating for the spurious frequency jumps in characterizing the oscillator steering voltage.  
     
     
       4. The method of claim  1 , further comprising: 
       detecting spurious frequency jumps in operating the oscillator independent of the reference clock; and  
       compensating for the spurious frequency jumps in calculating the holdover oscillator error.  
     
     
       5. The method of claim  2 , wherein operating the oscillator independent of the reference clock signal for a period of time further comprises providing a steering voltage to the oscillator that is extrapolated from the characterization of the oscillator steering voltage over time. 
     
     
       6. A method of testing stability of an oscillator, comprising: 
       tracking an oscillator clock steering voltage over a simulated holdover period of time; and  
       calculating a predicted holdover oscillator error from the tracked oscillator clock steering voltage.  
     
     
       7. The method of claim  6 , wherein calculating a predicted holdover oscillator error further comprises evaluating a rate of change of the oscillator clock steering voltage over time. 
     
     
       8. The method of claim  6 , further comprising: 
       detecting spurious frequency jumps during the simulated holdover period of time; and  
       compensating for the spurious frequency jumps in calculating the predicted holdover oscillator error.  
     
     
       9. A method of detecting deterioration of a crystal oscillator, comprising: 
       testing the stability of the crystal oscillator relative to a reference clock signal periodically;  
       storing the results of the periodic tests; and  
       comparing the results of the periodic tests over time.  
     
     
       10. The method of claim  9 , wherein comparing the results of the periodic tests comprises calculating whether the oscillator is becoming more stable or less stable over time. 
     
     
       11. The method of claim  10 , wherein comparing the results of the periodic tests further comprises calculating whether the rate at which the oscillator is changing in stability is increasing or decreasing. 
     
     
       12. The method of claim  9 , wherein comparing the results of the periodic tests comprises calculating whether the rate at which the oscillator is changing in stability is increasing or decreasing. 
     
     
       13. A clock module, comprising: 
       an oscillator that produces an oscillator clock signal;  
       a controller that is operable to characterize the oscillator stability relative to a reference clock signal by operating the oscillator independent of the reference clock signal for a period of time and further by calculating an holdover oscillator error that is the difference between the oscillator clock signal change and the reference clock signal change over the period of time, and that is further operable to compensate for the holdover oscillator error in controlling the oscillator.  
     
     
       14. The clock module of claim  13 , wherein the controller is further operable to detect spurious frequency jumps and to compensate for the spurious frequency jumps in the oscillator while characterizing the oscillator stability. 
     
     
       15. The clock module of claim  13 , further comprising a tracking module that is operable to track a difference between the reference clock signal and the oscillator clock signal. 
     
     
       16. The clock module of claim  15 , wherein the tracking module comprises a phase detector. 
     
     
       17. The clock module of claim  16 , wherein the tracking module further comprises a frequency divider. 
     
     
       18. An oscillator module, comprising: 
       a reference clock signal source;  
       a phase detector connected to the reference clock signal source and an oscillator, and further operable to detect a phase difference between the reference clock signal and an oscillator signal;  
       a filter connected to an output of the phase detector that provides a steering voltage signal to the oscillator to cause the oscillator signal to track the reference clock signal source; and  
       a controller that operates the oscillator independent of the reference clock signal with a predicted steering voltage for a simulated holdover period of time, and that further calculates a predicted holdover error that is the difference between the oscillator signal and the reference clock signal at the end of the simulated holdover period of time.  
     
     
       19. The oscillator module of claim  18 , wherein the filter and the controller are implemented in a Digital Signal Processor (DSP). 
     
     
       20. The oscillator module of claim  18 , wherein the controller is further operable to detect spurious frequency jumps and to compensate for the spurious frequency jumps in tracking the steering voltage. 
     
     
       21. A method of providing accurate timing signals when a reference timing signal is unavailable, the method comprising: 
       correcting an oscillator in accordance with the reference timing signal when such signal is available;  
       characterizing the oscillator based on a difference between the reference timing signal and an oscillator output signal;  
       compensating for spurious frequency jumps while characterizing the oscillator; and  
       operating the oscillator in accordance with the characterization when the reference timing signals are not available.